The present invention relates to a sublimer-reactor system for reducing a Group IVB metal chloride with magnesium.
Group IVB metals, including zirconium and hafnium, commercially are produced from sands by Kroll reduction processes where the sands are carbochlorinated to produce the metals in the form of chlorides and primarily tetrachlorides. The chlorides are batch fed to sublimers which vaporize the chlorides. The vapors then flow into reactors where they are reduced with magnesium to form a metal sponge in magnesium chloride. U.S. Pat. Nos. 4,511,399; 4,613,366 and 4,897,116 disclose the structure and operation of known sublimer-reactor systems, which are incorporated by reference. These systems may be employed to produce any Group IVB metal.
The prior art Kroll processes normally employ substantial amounts of excess magnesium to assure the complete reduction of the feed. This practice undesirably extends the cycle time of the Kroll processes because the excess magnesium requires additional melting time. In addition the use of excess magnesium commercially requires that the unreacted magnesium be recovered and recycled in order to reduce raw material costs. Recycled magnesium introduces water vapor and adsorbed contaminating gases into the feed, which must be desorbed in a degassing step before the reaction proceeds. U.S. Pat. No. 4,511,399 discloses the use of a level sensing system in a continuously fed sublimer to measure the chlorides, but even this system employs substantial amounts of excess magnesium which flow from a reactor through a magnesium tap valve to a recovery vessel.
Level sensors cannot be employed in sublimers used in case of zirconium or hafnium production with sufficient precision to measure the chlorides because the surface of the powder may not be level. This is caused by the tendency of the powders adjacent the heating surfaces of the sublimers to vaporize before the bulk of the powder, which may develop a cone shape. Thus the measured level is merely an approximation.
The prior art Kroll processes also employ extended and uncontrolled reduction steps where the exthermic reaction causes the temperature to reach &gt;950.degree. C. This step is taken to assure the complete reduction of the chlorides. It is believed that this practice permits substantial amounts of iron due to higher solubility of iron in magnesium at &gt;900.degree. C. Iron will diffuse from the reactor crucible containing the sponge into the sponge. It is believed that the best reaction temperature should be controlled between 800.degree.-900.degree. C. to produce higher quality zirconium, hafnium and the like (metal sponge).